c++: implement DR1363 and DR1496 for __is_trivial [PR85723]

c++: implement DR1363 and DR1496 for __is_trivial [PR85723]

is_trivial was introduced in
<https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2230.html>
which split POD into is_trivial and is_standard_layout.

Later came CWG 1363.  Since

  struct A {
    A() = default;
    A(int = 42) {}
  };

cannot be default-initialized, it should not be trivial, so the definition
of what is a trivial class changed.

Similarly, CWG 1496 concluded that

  struct B {
    B() = delete;
  }:

should not be trivial either.

P0848 adjusted the definition further to say "eligible".  That means
that

  template<typename T>
  struct C {
    C() requires false = default;
  };

should not be trivial, either, since C::C() is not eligible.

Bug 85723 reports that we implement none of the CWGs.

I chose to fix this by using type_has_non_deleted_trivial_default_ctor
which uses locate_ctor which uses build_new_method_call, which would
be used by default-initialization as well.  With that, all __is_trivial
problems I could find in the Bugzilla are fixed, except for PR96288,
which may need changes to trivially-copyable, so I'm not messing with
that now.

I hope this has no ABI implications.  There's effort undergoing to
remove "trivial class" from the core language as it's not really
meaningful.  So the impact of this change should be pretty low except
to fix a few libstdc++ problems.

	PR c++/108769
	PR c++/58074
	PR c++/115522
	PR c++/85723

gcc/cp/ChangeLog:

	* class.cc (type_has_non_deleted_trivial_default_ctor): Fix formatting.
	* tree.cc (trivial_type_p): Instead of TYPE_HAS_TRIVIAL_DFLT, use
	type_has_non_deleted_trivial_default_ctor.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wclass-memaccess.C: Add dg-warning.
	* g++.dg/ext/is_trivial1.C: New test.
	* g++.dg/ext/is_trivial2.C: New test.
	* g++.dg/ext/is_trivial3.C: New test.
	* g++.dg/ext/is_trivial4.C: New test.
	* g++.dg/ext/is_trivial5.C: New test.
	* g++.dg/ext/is_trivial6.C: New test.